Mercruiser 4.3 Set idle or timing first

[petedw]

I have a 1989 Mercruiser 4.3 with a thunderbolt IV ignition. I have a misfire at high rpm's (4200-4800). I checked out the fuel system and believe that it is fine. I replaced spark plugs, wires, cap, and rotor. The compression is good. The boat is idling at 900-1100 rpm's. I decided to take a look at the timing. It is sitting at close to 12BTDC. However, i'm not sure if that is correct because the idle will not go to the correct range. So, do I need to somehow get the idle down before checking the timing or assume that the reason the idle is high is partly due to timing advance. Thanks for any suggestions. Also, the timing mark is jumping around some when I check it. What does that indicate?

 

[Don S]

Re: Mercruiser 4.3 Set idle or timing first

It can be a back and forth type deal. When you are done, you need X? BTDC @ idle rpm.

Since you are at 900 rpm, slow it down to specs and recheck. If the timing is off, adjust it. If the rpm is then to low or two high adjust it and recheck the timing again.